Lamp flashing circuit

ABSTRACT

A circuit for flashing a xenon or other high power flash lamp employs a capacitor incrementally charged from a non-saturating inductor storing energy supplied at a low dc voltage. A transistor switches current to the inductor. The transistor is controlled by a transformer having a primary connected across the inductor. At the beginning of each capacitor charging cycle the transistor is turned on by regenerative action through the transformer. Current is supplied to the inductor until the transformer saturates, causing transistor turn-off. Energy stored in the inductor then is transferred to the capacitor via a diode. A reverse current coupled through the transformer maintains the transistor off until substantially all energy stored in the inductor has been transferred to the capacitor. The charging cycle repeats until the voltage across the capacitor is sufficient to fire the lamp.

United States Patent [191 Leskin Nov. 13, 1973 LAMP FLASHING CIRCUITMorton B. Leskin, 4046 Cody Rd., Sherman Oaks, Calif. 91403 [22] Filed:May 25, 1972 [2]] Appl. No.: 256,838

[76] Inventor:

Primary ExaminerRoy Lake Assistant Examiner Lawrence J. DahlAttorney-Fred Flam et al.

[57] ABSTRACT A circuit for flashing a xenon or other high power flashlamp employs a capacitor incrementally charged from a non-saturatinginductor storing energy supplied at a low dc voltage. A transistorswitches current to the inductor. The transistor is controlled by atransformer having a primary connected across the inductor. At thebeginning of each capacitor charging cycle the transistor is turned onby regenerative action through the transformer. Current is supplied tothe inductor until the transformer saturates, causing transistorturn-off. Energy stored in the inductor then is transferred to thecapacitor via a diode. A reverse current coupled through the transformermaintains the transistor off until substantially all energy stored inthe inductor has been transferred to the capacitor. The charging cyclerepeats until the voltage across the capacitor is sufficient to fire thelamp.

14 Claims, 2 Drawing Figures I I4 Ida N19 -v L PATENTEU NM 13 1915 m m2.Op

WQORH OF .N i m w Wm 1Q mm (A mm mm R, c2 'W E II o =N LAMP FLASHINGcmcurr BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a lamp flashing circuit, and particularly to a highefficiency circuit for incrementally charging a capacitor to a highvoltage.

2. Description of the Prior Art For many applications it is desirable tooperate a xenon or other high power flash lamp from a low voltage dcsupply. One such use is in a building security system having a burglaralarm connected to a police station or a private security agency. A highpower flash lamp is mounted atop the building and connected to flashwhen the burglar alarm is tripped. The flashing light, visible at somedistance, allows a police helicopter or patrol car to spot the buildingreadily at night. Chances of apprehending the thief are improved.Battery operation is desirable to insure lamp flashing even if the thiefshould disable ac power to the building.

High intensity xenon flash lamps operate at a relatively high voltage,usually above 200 volts. Circuitry must be provided to convert a low dcsupply to a voltage sufficient to fire the lamp. Typically this is doneby repetitively, inductively storing energy at battery voltage, thentransferring the energy from the inductor to the capacitor. In this way,the capacitor is charged incrementally to a level sufficient to fire theflash lamp. The problem is one of efficiency in the energy transferoperation. In prior art circuits efficiencies below 50 percent are therule. Much of the energy not transferred to the capacitor is lost asheat, so that thermal dissipation from the circuit package becomes aserious consideration. More important, the low efficiency severlyshortens the time period during which flashing can be powered with acertain battery. For example, if 19 watts are required to flash the lamponce per second,

a 24 volt, 6 ampere hour battery will flash the lamp once per second fora period of only 1.5 hours at 40 percent efficiency. One object of thepresent invention is to provide a lamp flashing circuit having higherefficiency, typically greater than 75 percent, with concomitant lowerheat loss and longer flashing time for a particular battery than hasbeen possible in the past.

Other prior art problems relate to the manner of incrementally chargingthe capacitor. In one type of circuit'the battery is connected to theinductor by an electronic switch. The capacitor and a diode areconnected across the switch. With the switch closed, current is storedin the inductor. At the instant when the current reaches a preselectedvalue, the switch is opened. As the magnetic field of the inductorcollapses, the stored energy is conducted via the diode to thecapacitor. Although simple in concept, the circuit is difficult toimplement since the switch must be operated at a precise instant,requiring special circuitry to sense current through the inductor.Furthermore, a diode having very fast turn-off speed is required. If thediode does not turn off rapidly when energy transfer to the capacitor iscompleted, current may flow back through the diode and appear across theswitch, which may be closing for the next cycle. Efficiency is reduced,and catastrophic damage to a semiconductor switch may result.

Another prior art approach involves the use of a blocking oscillatorhaving a transformer which functions both as a feedback element and forenergy storage. The oscillator cycle is established by saturation ofciency is limited by compliance voltage considerations to less thanabout 50 percent.

Thus it is another object of the present invention to provide acapacitor charging circuit which does not require a diode having fastturn-off time, and wherein the inductor used for energy storage isoperated at a nonsaturating level to achieve optimum energy transferefficiency.

SUMMARY OF THE INVENTION These and other objects are achieved byproviding a lamp flashing circuit wherein current to an energy storageinductor is switched by a transistor. Conduction of the transistor iscontrolled by a transformer having a primary connected across theinductor. At the beginning of each capacitor charging cycle thetransistor is turned on by regenerative action through the transformer.The transistor remains on until the transformer saturates. During thison time, current is supplied to the inductor, which does not saturate.When the transistor switches off, the inductively stored energy iscoupled via a diode to the capacitor. A reverse current coupled throughthe transformer maintains the transistor off until the inductor hascompletely discharged, completing the charging cycle. The action repeatsuntil the voltage across the capacitor is sufficient to fire the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS A detailed dscription of the inventionwill be made with reference to the accompanying drawings, wherein:

FIG. 1 is an electrical schematic diagram of a preferred embodiment ofthe inventive lamp flashing circuit.

FIG. 2 is a fragmentary electrical schematic diagram of a simplifiedversion of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detaileddescription is of the best presently contemplated modes of carrying outthe invention. This description is not to be taken in a limiting sense,but is made merely for the purpose of illustrating the generalprinciples of the invention since the scope of the invention best isdefined by the appended claims.

Operationalcharacteristics attributed to forms of the invention firstdescribed also shall be attributed to forms later described, unless suchcharacteristics obviously are inapplicable or unless specific exceptionis made.

Referring now to FIG. 1, the inventive lamp flashing circuit 10 ispowered by a battery or other dc supply connected across the terminals11a, 11b. Energy from this supply is used to charge a capacitor l2to avoltage sufficient to fire a xenon or other flash lamp 13. Such chargingis accomplished incrementally by storing energy in a non-saturatinginductor 14 during an interval when a transistor 15 is biased on bycircuitry including a transformer 16. The energy stored in the inductorl4 then is transferred via a diode 17 to the capacitor 12.

The transistor 15 is held off during the entire energy transferduration, as described below, so that the diode 17 need not exhibit fastturn-off characteristics.

At the beginning of each capacitor 12 charging cycle no energy is storedin the inductor 14 and the transformer 16 is not saturated. Turn-on ofthe transistor 15 is achieved by regenerative action involving feedbackthrough the transformer 16. To this end, a starting voltage is suppliedby a resistor 19 to the base of a low power transistor 20 via a pathincluding the supply terminal 110, a line 21, the resistor 19, thesecondary 16s of the transformer 16 and a current limiting resistor 22.

The collector of the transistor 20 is connected directly to a tap on theinductor 14. The transistor 20 emitter is connected via a resistor 23and a line 24 to the other supply terminal 11b, and via a resistor 25 tothe base of the transistor 15. Thus the voltage supplied via theresistor 19 causes the transistor 20 to begin conduction, driving thebase of the transistor 15 positive. The transistor 15 itself starts togo on, tending to clamp the collector and the line 26 toward thenegative potential of the terminal 11b.

Accordingly, a current path is provided from the positive terminal 11avia the line 21 the primary 16p of the transformer 16, a currentlimiting resistor 27 and the collector-emitter of the transistor 15 tothe negative terminal 11b. The resultant current flow through thetransformer primary 16p induces in the secondary 16s a signal whichenhances the positive starting voltage at the transistor 20 base. Thusthe transistor 20, and hence the transistor 15 both are driven furtherinto conduction. This regenerative action causes the transistor lrapidly to reach the condition of maximum conduction.

When the transistor conducts, current is supplied to the inductor 14from the dc supply via the collectoremitter path of the transistor 15.This current causes energy to be stored in the form of a magnetic fieldin the inductor 14. Current flow through the transistor 15, andconcomitant energy storage in the inductor l4, continues until thetransformer 16 saturates. The parameters of the transformer 16 and theinductor 14 are selected so that such transformer 16 saturation occurswhile the inductor 14 is still unsaturated.

When the transformer 16 saturates, transformer action ceases and apositive voltage no longer is induced in the secondary 16s. Thetransistor base voltage decreases correspondingly, and the bias suppliedvia the resistor 19 is insufficient by itself to keep the transistor 20on. As a result, the transistor 20 turns off, causing the transistor 15base to go negative and the transistor 15 to turn off. This opens thecurrent path through the transformer primary 16p, inducing in thesecondary 16s a negative going pulse which reinforces turn-off of thetransistors 20 and 15.

As soon as the transistor 15 turns off, current no longer is supplied tothe inductor 14. The magnetic field in the inductor 14 starts tocollapse, and the energy stored therein is inductively coupled to awinding 14a and thence via the diode 17 to the capacitor 12.Substantially all the energy stored in the inductor 14 is transferred tothe capacitor 12.

As the inductor 14 magnetic field collapses, a reverse current appearsacross the inductor 14. This current also flows through the transformerprimary 16p to induce in the secondary 16s a voltage which is negativeat the base of the transistor 20. The secondary 16s current flow isthrough the path including the resistor 22, the base-emitter path of thetransistor 20, the resistor 23, the line 24 and a diode 28 shunted by acapacitor 29. As a result, the transistor 20, and hence the transistor15, is clamped off so long as the inductor 14 is discharging. Thetransistor 15 will not turn on until substantially all of the energystored in the inductor 14 has been transferred to the capacitor 12. Thediode 17 need not exhibit fast turn-off time, since it is the dischargeof the inductor 14 which determines how long the transistor 15 isclamped off irrespective of the diode 17 characteristics.

When the inductor 14 has discharged completely, current flow momentarilywill cease through the transformer 16. This completes the charting cycleand returns the circuit 10 to the initial condition for the nextcapacitor charging cycle, now initiated by the starting signal suppliedvia the resistor 19. Note that the diode 28 polarity prevents flowtherethrough of this turn-on signal.

A capacitor 31 filters the supply voltage, and a Zener diode 32 protectsthe transistor 15 from damage by high voltage spikes which may occur inthe circuit 10.

Eventually the voltage across the capacitor 12 will become sufficient tofire the flash lamp 13. A circuit 10a detects when this voltage levelhas been reached and provides a pulse on a line 35 to ignite the flashlamp 13. To this end, a voltage divider including three resistors 36,37, 38 is connected across the capacitor 12. As the capacitor 12charges, the voltage developed across the resistors 37, 38 is used tocharge a capacitor 39 and another capacitor 40 is charged by the voltageacross the resistor 38.

When the voltage across the capacitor 12 reaches a lamp flashing level,the capacitor 40 will be charged sufficiently to actuate a trigger diode41 and discharge via a resistor 42. The resultant current will trigger asilicon controlled rectifier (SCR) 43. As a result, the capacitor 39rapidly will discharge through the SCR 43 and the primary of an ignitiontransformer 44. The current pulse induced in the transformer 44secondary is supplied via the line 35 to ignite the flash lamp 13, whichflashes as the capacitor 12 discharges. The next flashing cycle thenbegins.

If for some reason the lamp 13 should not flash, the charge on thecapacitor 12 might continue to increase to the level of capacitordestruction. To prevent this, a protector circuit 10b inhibits chargingoperation when the voltage across that capacitor exceeds the level atwhich the lamp 13 is set to flash. 1

The circuit 10b uses a voltage divider comprising two resistors 46, 47connected across the capacitor 12. Should the charge on the capacitor 12exceed the lamp firing level, the voltage at the junction of theresistors 46, 47 will be sufficient to gate on an SCR 48 via a triggerdiode 49. Conduction of the SCR 48 effectively clamps the base of thetransistor 20 to the negative supply terminal llb, thereby preventingturn-on of the transistor 20. This clamps off the transistor 15 inhibiting further energy storage by the inductor 14, thereby terminating thecapacitor 12 charging operation.

In the simplified circuit of FIG. 2 the transformer secondary 16s isconnected directly (or via a current limiting resistor not shown) to thetransistor 15 base. The transistor 20 is eliminated. The transistor 15must be capable of switching the relatively high current to the inductor14 and have relatively high gain. In the FIG. 1 embodiment, the controlsignals are effectively amplified by the low power transistor 20, sothat the transistor 15 need not exhibit high gain.

The circuit of FIG. 1 exhibits high efficiency, typically on the orderof 80 percent. Thus with a 12 volt battery supplying 24 watts (2amperes) to the terminals 11a, 11b, the capacitor 12 may be charged onceper second to 19 watts. Using a flash lamp '13 rated at 19 joules, thecircuit thus could flash the lamp once per second. With a 6 ampere-hourbattery, such flashing could continue for three hours.

By way of example only, the inductor l4 may comprise a coil of nineteenturns, with a tap at sixteen turns, wound on a toroidal core ofmolybdenum alloy powder material. The secondary winding 14a may comprisea coil of about 235 turns wound on the same core. The transformer 16 mayhave a primary of 100 turns wound on a ferrite toroidal core alsocontaining a secondary of about 50 turns.

As an alternative to using the secondary winding 14a for coupling energyto the capacitor 12, the diode 17 may be connected directly to theinductor 14. However with such arrangement, the transistor should becapable of withstanding a voltage equal to that developed across thecapacitor 12. Using the secondary winding 140 as shown, the transistor15 need only withstand a voltage equal to that across the capacitor 12times the secondary l4a-to-inductor 14 turns ratio.

Intending to claim all novel, useful and unobvious features shown ordescribed, the applicant claims:

1. A lamp flashing circuit powered by a dc supply and including acapacitor charged incrementally during successive charging cycles to alevel sufficiently high to flash said lamp, comprising:

a semiconductor switching device,

an inductor connected in series with said device across said supply,

means for coupling energy stored in said inductor to said capacitor asthe field of said inductor collapses,

a control transformer having a primary connected in parallel with saidinductor and a secondary operatively connected to control said switchingdevice,

circuit means including said transformer secondary for regenerativelyturning on said switching device at the beginning of each charging cycleto permit current flow to said inductor for energy storage therein,

said control transformer saturating before saturation of said inductor,saturation of said transformer causing turn-off of said switch device toterminate said current flow to said inductor, the resultant collapse ofsaid inductive field inducing a reverse polarity signal in saidtransformer secondary which maintains said switching device off untilsubstantially all energy stored in said inductor has been coupled tosaid capacitor.

2. A lamp flashing circuit according to claim 1 wherein said powerswitching device comprises a first transistor and where said circuitmeans comprises:

a circuit connecting one terminal of said transformer secondary to thebase of said first transistor,

a first diode connecting the other terminal of said transformersecondary to a terminal of said supply, and

a resistor connected between said transformer secondary other terminaland the other terminal of said supply to provide, in conjunction withth'e output of said transformcr,a signal regeneratively initiatingturn-on of said first transistor. y I

3. A lamp flashing circuit according to claim 2 wherein said circuitconnecting comprises:

a second transistor having a base connected to said transformersecondary one terminal and operatively connected to turn on said firsttransistor when said second transistor is on.

4. A lamp flashing circuit according to claim 3 wherein one non-controlelement of said second transistor is connected to a tap on said inductorand wherein the other non-control element is resistor connected to aterminal of said supply and to the base of said first transistor.

5. A lamp flashing circuit according to claim 1 wherein said means forcoupling comprises:

a secondary winding inductively coupled to said inductor, and

unidirectional current flow'means connecting said secondary winding tosaid capacitor.

6. A lamp flashing circuit according to claim 5 further comprising:

voltage divider means for sensing when the voltage across said capacitoris sufficient to flash said lamp,

and

ignitor means cooperating with said voltage divider means for ignitingsaid lamp when sufficient capacitor voltage is sensed.

7. A lamp flashing circuit according to claim 6 further comprising:

means for inhibiting the incremental charging of said capacitor shouldthe voltage across said capacitor substantially exceed said sufficientvoltage.

8. A circuit powered by a low voltage dc source for incrementallycharging a capacitor during successive charging cycles to a highvoltage, comprising:

an inductor,

a transistor connected in series with said inductor across said sourceto provide current from said source to said inductor when saidtransistor is on,

a transformer having a primary connected across said inductor and asecondary, said transformer saturating prior to said inductor,

circuit means connecting one tenninal of said transformer secondary tothe control electrode of said transistor,

bias supply means cooperating with current induced in said transistorsecondary regeneratively to drive said transistor into conduction at thebeginning of each charging cycle, conduction of said transistor causingcurrent flow to said inductor with concomitant inductive energy storagetherein, until saturation of said transformer causes said transistor toturn off, thereby terminating current flow to said inductor,

energy transfer means for transferring the stored energy from saidinductor to said capacitor as the magnetic field in said inductorcollapses subsequent to turn-off of said transistor,

reverse current induced in said transformer secondary as said inductorfield collapses maintaining said transistor off while substantially allof the energy stored in said inductor is transferred to said capacitor.

capacitor.

[0. A circuit according to claim 9 wherein the reverse current resultantas said inductor field collapses produces via said transformer asignalmaintaining said transistor off, said circuit including a seconddiode op eratively connected to permit flow of said signal.

11. A circuit according to claim 10 wherein said second diode and thesecondary of said transformer are connected in series between oneterminal of said source and a transistor base, and wherein said biassupply means comprises a resistor connected from the other terminal ofsaid source to the junction of said second diode and said transformersecondary, and wherein said transformer produces prior to saturation asignal enhancing conduction of the transistor providing current to saidinductor.

12. A circuit according to claim 11 wherein said transformer secondaryis connected tothe base of the transistor providing current to saidinductor.

13. A circuit according to claim. together with a second transistorconnected to control conduction of said transistor providing current tosaid inductor, and wherein said transformer secondary is connected tothe base of said second transistor. v

14. A circuit according to claim 8 together with a flash lamp connectedto be powered by the energy stored in said capacitor, and means forigniting said lamp when thevoltage across said capacitor reaches a valuesufficient to flash said lamp.

1. A lamp flashing circuit powered by a dc supply and including acapacitor charged incrementally during successive charging cycles to alevel sufficiently high to flash said lamp, comprising: a semiconductorswitching device, an inductor connected in series with said deviceacross said supply, means for coupling energy stored in said inductor tosaid capacitor as the field of said inductor collapses, a controltransformer having a primary connected in parallel with said inductorand a secondary operatively connected to control said switching device,circuit means including said transformer secondary for regenerativelyturning on said switching device at the beginning of each charging cycleto permit current flow to said inductor for energy storage therein, saidcontrol transformer saturating before saturation of said inductor,saturation of said transformer causing turn-off of said switch device toterminate said current flow to said inductor, the resultant collapse ofsaid inductive field inducing a reverse polarity signal in saidtransformer secondary which maintains said switching device off untilsubstantially all energy stored in said inductor has been coupled tosaid capacitor.
 2. A lamp flashing circuit according to claim 1 whereinsaid power switching device comprises a first transistor and where saidcircuit means comprises: a circuit connecting one terminal of saidtransformer secondary to the base of said first transistor, a firstdiode connecting the other terminal of said transformer secondary to aterminal of said supply, and a resistor connected between saidtransformer secondary other terminal and the other terminal of saidsupply to provide, in conjunction with the output of said transformer, asignal regeneratively initiating turn-on of said first transistor.
 3. Alamp flashing circuit according to claim 2 wherein said circuitconnecting comprises: a second transistor having a base connected tosaid transformer secondary one terminal and operatively connected toturn on said first transistor when said second transistor is on.
 4. Alamp flashing circuit according to claim 3 wherein one non-controlelement of said second transistor is connected to a tap on said inductorand wherein the other non-control element is resistor connected to aterminal of said supply and to the base of said first transistor.
 5. Alamp flashing circuit according to claim 1 wherein said means forcoupling comprises: a secondary winding inductively coupled to saidinductor, and unidirectional current flow means connecting saidsecondary winding to said capacitor.
 6. A lamp flashing circuitaccording to claim 5 further comprising: voltage divider means forsensing when the voltage across said capacitor is sufficient to flashsaid lamp, and ignitor means cooperating with said voltage divider meansfor igniting said lamp when sufficient capacitor voltage is sensed.
 7. Alamp flashing circuit according to claim 6 further cOmprising: means forinhibiting the incremental charging of said capacitor should the voltageacross said capacitor substantially exceed said sufficient voltage.
 8. Acircuit powered by a low voltage dc source for incrementally charging acapacitor during successive charging cycles to a high voltage,comprising: an inductor, a transistor connected in series with saidinductor across said source to provide current from said source to saidinductor when said transistor is on, a transformer having a primaryconnected across said inductor and a secondary, said transformersaturating prior to said inductor, circuit means connecting one terminalof said transformer secondary to the control electrode of saidtransistor, bias supply means cooperating with current induced in saidtransistor secondary regeneratively to drive said transistor intoconduction at the beginning of each charging cycle, conduction of saidtransistor causing current flow to said inductor with concomitantinductive energy storage therein, until saturation of said transformercauses said transistor to turn off, thereby terminating current flow tosaid inductor, energy transfer means for transferring the stored energyfrom said inductor to said capacitor as the magnetic field in saidinductor collapses subsequent to turn-off of said transistor, reversecurrent induced in said transformer secondary as said inductor fieldcollapses maintaining said transistor off while substantially all of theenergy stored in said inductor is transferred to said capacitor.
 9. Acircuit according to claim 8 together with a winding inductively coupledto said inductor and a diode connecting said winding to said capacitor,collapse of the magnetic field in said inductor resulting in energytransfer via said winding and said diode to said capacitor.
 10. Acircuit according to claim 9 wherein the reverse current resultant assaid inductor field collapses produces via said transformer a signalmaintaining said transistor off, said circuit including a second diodeoperatively connected to permit flow of said signal.
 11. A circuitaccording to claim 10 wherein said second diode and the secondary ofsaid transformer are connected in series between one terminal of saidsource and a transistor base, and wherein said bias supply meanscomprises a resistor connected from the other terminal of said source tothe junction of said second diode and said transformer secondary, andwherein said transformer produces prior to saturation a signal enhancingconduction of the transistor providing current to said inductor.
 12. Acircuit according to claim 11 wherein said transformer secondary isconnected to the base of the transistor providing current to saidinductor.
 13. A circuit according to claim 11 together with a secondtransistor connected to control conduction of said transistor providingcurrent to said inductor, and wherein said transformer secondary isconnected to the base of said second transistor.
 14. A circuit accordingto claim 8 together with a flash lamp connected to be powered by theenergy stored in said capacitor, and means for igniting said lamp whenthe voltage across said capacitor reaches a value sufficient to flashsaid lamp.